Endothelial Cell Death Induced by Tumor Necrosis Factor-α Is Inhibited by the Bcl-2 Family Member, A1

Abstract

Endothelial cells play a central role in the inflammatory process. Tumor necrosis factor-␣ (TNF) is a multifunctional cytokine which elicits many of the inflammatory responses of endothelial cells. While TNF directly causes apoptosis of tumor cells and virally infected cells, normal cells are generally resistant. However, most resistant cells, including human endothelial cells, can be rendered susceptible to TNF by inhibiting RNA or protein synthesis. This finding suggests that TNF provides a cell survival signal in addition to a death signal. We have previously cloned a human Bcl-2 homologue, A1, and shown that it is specifically induced by proinflammatory cytokines but not by endothelial growth factors. In this study, we show that retroviral-mediated transfer of the A1 cDNA to a human microvascular endothelial cell line provides protection against cell death initiated by TNF in the presence of actinomycin D. The induction of A1 by TNF in this system is mediated via a protein kinase C pathway. Since TNF signaling has also been shown to proceed via ceramides, we tested whether exogenous ceramides could induce A1. Our findings indicate that ceramides do not induce A1 but do up-regulate c-jun and induce endothelial death. Ceramide-activated endothelial death is also inhibited by A1, suggesting that TNF may initiate divergent survival and death pathways via separate lipid second messengers. TNF1 is an inflammatory cytokine originally defined by its tumoricidal activity (1, 2). Subsequently, it has been shown to evoke multiple biological responses affecting virtually every cell type (3). Considerable attention has recently been paid to the apoptotic pathway elicited by TNF. A series of elegant experiments have defined a death pathway emanating from the TNF receptor 1. Engagement of the TNF receptor results in cell death by recruitment of a complex of proteins to the cell membrane including TRADD, FADD/Mort1, and MACH/FLICE which culminates in the activation of cysteine proteases (4 -10). TNF stimulation also results in signal transduction pathways mediated by two lipid second messengers, diacylglycerol and ceramide (3). While ceramide is a transducer of TNF-induced apoptosis (11, 12), diglyceride or phorbol ester pretreatment can protect against ceramide-induced cell death (11, 13). Ceramides appear to act downstream of FADD/Mort1 since a FADD dominant-negative can block TNF-induced apoptosis but not death mediated by ceramide (7).Although tumor cells and virally infected cells are susceptible to TNF-induced death, many normal cells are not (1, 2). In this respect, human endothelial cells which play a pivotal role in modulating the inflammatory response are not directly killed by TNF (14). However, it has previously been shown that in the presence of RNA or protein synthesis inhibitors, endothelial cells as well as other cells can be rendered sensitive to TNF (14 -16). Conversely, it has also been demonstrated that sensitive cells can be made resistant to TNF challenge by prior sublethal exposure to TNF (15, 16). Th...

Lipopolysaccharide Signals an Endothelial Apoptosis Pathway Through TNF Receptor-Associated Factor 6-Mediated Activation of c-Jun NH2-Terminal Kinase

“…A similar phenomenon has been described for TNF (6). While exposure to TNF does not cause endothelial apoptosis, the combination of TNF and CHX or actinomycin D does (6). In both cases, the concomitant induction of antiapoptotic genes has been shown to inhibit the apoptotic pathway that is activated (4,6).…”

“…In contrast, when the expression of new genes is inhibited using cycloheximide (CHX) 3 or actinomycin D, endothelial cells become sensitized to the apoptotic signals evoked by LPS (3)(4)(5). A similar phenomenon has been described for TNF (6). While exposure to TNF does not cause endothelial apoptosis, the combination of TNF and CHX or actinomycin D does (6).…”

Constitutive proteasome-mediated turnover of Bfl-1/A1 and its processing in response to TNF receptor activation in FL5.12 pro-B cells convert it into a prodeath factor

“…18,26,[27][28][29] However, its protective activity is not always absolute, as Bfl-1/A1 is less protective than Bcl-xL in microvascular endothelial cells treated with tumor necrosis factor (TNF)a and only delays TNFa-induced apoptosis in HT1080 fibrosarcoma cells. 25,30 Similarly, while Bfl-1 potently blocks etoposideinduced cell death in HT1080 fibrosarcoma cells, it is much less effective against this agent in Jurkat T-cells. 25,28 These results suggest that the level of protection conferred by Bfl-1 is perhaps subject to modulation in different cell contexts and in response to particular apoptotic stimuli.…”

Bcl2a1 serves as a switch in death of motor neurons in amyotrophic lateral sclerosis

“…Bcl2a1 has protective effects in some experimental paradigms but promotes programmed cell death in others. 11 Furthermore, it responds to oxidative stress, is a direct transcriptional target of nuclear factor (NF)-kB 12,13 and is induced by the inflammatory cytokines tumour necrosis factor (TNF-a) and interleukin (IL)-1b 14,15 These unique characteristics of Bcl2a1 prompted us to study the role of this protein in ALS, as reactive oxygen species, NF-kB, TNF-a and IL-1b have been proposed by us and by others to contribute in the molecular crosstalk occurring among different cell types in the pathogenesis of ALS 1,6 We have investigated expression of the Bcl2 family members in spinal cords of transgenic G93A-SOD1, WT-SOD1 and non-transgenic mice. As shown in Figure 1, Bcl2a1 is the only member of this family to be upregulated in both asymptomatic and symptomatic G93A mice (Figure 1a), with a tendency to decrease in the final stages of disease.…”

“…It has been shown that Bcl2a1 has various effects in response to TNF-a in different cell cultures: it acts as an antiapoptotic in immune-derived cell lines, 13 in microvascular endothelial cells 20 and in HT1080 fibrosarcoma cells, 14 whereas it is clearly proapoptotic in B cells. 11 In this work, we demonstrate in different cell cultures (NSC-34, undifferentiated and differentiated ETNA cells and primary cultures from spinal cords) that the overexpression of Bcl2a1 and G93A-SOD1 increases the level of cell death after exposure to TNF-a, a situation mimicking the condition of neuroinflammation occurring in ALS.…”